Hearing aids are tested by supplying a known acoustical test stimulus to the hearing aid microphone and measuring the resulting output. Increasingly, modern hearing aids employ a combination of directional responding microphones and non-linear signal processing to provide better performance to the end-user. Because the non-linear circuitry often causes both the gain and the frequency response of the hearing aid to be level-dependent, it is not possible to measure an accurate directional response by using (for example) two sound sources, one front-facing (i.e. in front of the hearing aid) and the other rear-facing (i.e. facing the rear of the hearing aid), and separately in time sweeping them through various frequencies. An accurate measurement of the directional characteristic requires that the front-facing and rear-facing acoustical stimuli be presented simultaneously.
Traditionally, directional response testing for directional hearing aids has been performed in an anechoic test space in which the front-facing and rear-facing responses are measured separately, typically by making a front-facing measurement and then rotating the hearing aid 180° in the test space to make the rear-facing measurement. As mentioned, measuring the front-facing and rear-facing responses separately will introduce significant error if the hearing aid has level-dependent gain and frequency shaping circuitry that responds to the overall input level. As an example, the rear-facing signal may be attenuated by the directional microphone by upwards of 10 dB, so when this signal is presented in isolation, the level-dependent circuitry will adapt accordingly to this low-level signal. However, under real-world conditions, the front-facing signal will be present simultaneously with the rear-facing signal and will not be attenuated. This will result in a significantly higher total signal presented to the level-dependent circuitry and consequently the hearing aid will under these conditions have a different gain and frequency response.
Using an anechoic test space presents additional problems. Such space must be large and filled with sound absorbing material to prevent standing waves, and this makes it impractical for use by most hearing aid dispensers. In addition, the responses measured in an anechoic chamber do not accurately reflect the real world performance that might be expected in a typical hard-walled room such as in a home or office environment where standing waves are present. It has not previously been possible to assess the performance of a directional microphone system in a real world echoic environment because it has not been possible to present appropriate front-facing and rear-facing signals simultaneously.